1. Field of the Invention
The present invention is directed to a sensitivity measuring apparatus for measuring the sensitivity of a fire detector in a fire alarm system.
2. Description of the Related Art
The measurement of the sensitivity of a smoke type fire detector has been conventionally effected as follows: the fire detector is mounted on the ceiling of a room with its output terminal provided externally in an easily accessible manner. With the fire sensor left mounted, an output signal is received from the output terminal to be used for sensitivity measurement. Since, in this case, the dismounting of the fire detector from the ceiling or the like is not needed, the overall time required for sensitivity measurement is shortened.
The output signal from a smoke type fire detector is typically an impedance transformed chamber voltage in an ionization type fire detector. Another kind of output signal is typically a pulsed voltage signal in a photoelectric type fire detector (scattered light type fire detector) which comprises a detector chamber, a light emitting element and a light receiving element. In the photoelectric type fire detector, the light receiving element receives scattered light of the light output continually emitted from the light emitting element. An amplifier amplifies the output signal from the light receiving element to output pulsed voltage signals.
The output signal from the smoke fire detector varies with the smoke density. Even under a constant smoke density, the strength of the output signal varies due to dirt or the like deposited on the detector itself; that is, the sensitivity of the detector still varies. In the measurement of the sensitivity of the fire detector, a sensitivity measuring apparatus receives the output of the fire detector to display the output signal, for example, an output voltage while the smoke density is kept constant (normally the smoke density is kept at almost 0%/m). A reference table which lists sensitivity values of the fire detector versus the output voltages is prepared beforehand. Based on the value of the output voltage displayed by the sensitivity measuring apparatus, the corresponding sensitivity may be found on the reference table. The sensitivity of the fire detector is measured in this way.
The sensitivity measuring apparatus typically comprises an amplifier for processing the output signal of the fire detector, an analog signal processing circuit including a sample and hold circuit and the like, and an A/D converter for converting processed analog signals into digital signals. When the measurement results are output in the form of analog signals, a D/A converter for converting the digital signals into analog signals may be further required.
Since many buildings are already equipped with fire detectors and the buildings lie scattered in broad areas, a great number of fire detectors are accordingly produced and supplied. To maintain accuracy in measurements in the sensitivity measuring apparatus, the internal circuitry in each apparatus should be individually adjusted to keep it to within acceptable tolerance.
The related tolerance values are, for example, the amplification tolerance of an amplifier, the DC offset voltage tolerance of a sample hold circuit, and the reference voltage tolerances of an A/D converter and a D/A converter. Conventionally, each circuit has been adjusted to be within a required tolerance range by adjusting its built-in variable resistors. This adjustment is performed not only at the production of each sensitivity measuring apparatus but also every predetermined period of time to compensate for aging effects. This adjustment requires a great deal of manpower and time.
By the type of the output signal, fire detectors may be divided into several categories: for example, ionization type fire detectors, photoelectric type fire detectors, and other types of fire detectors. The sensitivity measuring apparatus thus contains several measuring circuits, each corresponding to a particular type of the output signal of the fire detector to be measured. In making measurements, an inspector needs to switch from one circuit to another in the sensitivity measuring apparatus to match the output signal type of the fire detector to be measured.
The inspector may judge the output signal type of the fire detector to be measured from the appearance of the fire detector or from the model name and other information on the label which the fire detector carries. Even if fire detectors may look similar, they may provide actually different types of output signals. After a long time of use, the content of the label which a fire detector carries may be illegible. In such a case, there is a possibility that the inspector may select an incorrect circuit.
If this happens, a large magnitude of error may be introduced into sensitivity measurement results of the fire detector. The inspector may judge a fire detector with proper sensitivity to be faulty. Conversely, the inspector may judge a fire detector with poor sensitivity to be normal.
Furthermore, when an inspector has completed sensitivity measurements of a fire detector using a sensitivity measuring apparatus, he may forget turning off power for the apparatus. If he forgets, internal batteries discharge, and the apparatus can not be used again.
To avoid such an inadvertent situation, there is a way in which power is forcibly turned off only when a predetermined period of time has elapsed since the sensitivity measuring apparatus was switched on. In this arrangement, however, the sensitivity measuring apparatus might be switched off even in the middle of measurement operation when the predetermined time has been elapsed, although the batteries are prevented from discharging attributable to an inspector's inadvertent omission of switching off operation. The measurement is thus interrupted, and the inspector needs to switch on again. This procedure is complex.
In the course of sensitivity measurement, if an external noise is received by the fire detector and is superimposed onto the detector's own output signal, the sensitivity measuring apparatus receives the sum of the external noise and the fire detector output signal, and performs an erroneous sensitivity judgment. If an external noise is directly received by the sensitivity measuring apparatus, it also performs an erroneous sensitivity judgment.